Edit Mode Dimensions
Keep in mind that when you add a primitive (such as a cube), you can only adjust its defining dimensions immediately after creating it.

Once you add the second cube you can find the controls to adjust its dimensions at the bottom of the left hand toolbar (accessed by pressing T), or by pressing F6.  As soon as you perform another action Blender "forgets" that the added mesh was a cube and doesn't consider its geometry any different than the rest of the mesh so from then on you can only adjust it's size by manipulating the geometry directly.